The Other section of the Store and Inventory contains collection assemblers, items for entering paid locations, and duplicate stickers.
Assemblers are used in the Collections window, and the items for entering locations are spent on playing in paid and event locations.
You can get them as rewards for quests, collections, and game events, in gifts and monster treasuries, or by completing locations and mini-games.
You can receive sticker duplicates as a gift from a friend or find them in sticker chests during the Sticker Fair and the Festival of Memories events.